1. Field of the Invention
This invention relates to methods and structures for accomplishing wafer passivation. and more particularly to methods and structures for accomplishing wafer passivation which employs multilevel films associated with a surface or termination of monocrystalline silicon.
2. Description of Related Art Including Information Disclosed under .delta..delta.1.97-1.99
The difficulty of surface charge control in monocrystalline silicon wafer processing, encapsulation, and device operation has long been a problem in the semiconductor industry. Surface charges or potentials affect device stability and reliability, and require continual process monitoring and adjustment. Various surface passivation techniques which have been advanced to address this problem, however, unsatisfactorily desensitize surface potentials to ionic, moisture, thermal, and stress effects.
Silicon dioxide passivation layers have been proposed, and are presently in widespread use, but have numerous disadvantages. Classical silicon dioxide passivation techniques adversely affect device performance by creating fixed positive charges near the siliconsilicon dioxide interface which alter the surface Fermi potentials. Silicon dioxide films are also sensitive to ionic redistribution, particularly to cations like sodium, and exhibit a strong reflection of surface charge onto the monocrystalline surface.
Additionally, moisture penetration of the silicon dioxide tends to enhance electron trapping, exacerbating hot carrier charge injection into the dielectric. Because silicon dioxide is deposited at relatively high temperatures, the silicon-silicon dioxide band structure (and hence the surface Fermi potential) is very sensitive to mechanical and thermal stresses: therefore, devices exhibit a piezoelectric sensitivity. The high temperature processing and the oxygen vacancies perturb the monocrystalline periodicity near the surface, which degrades the device boundary recombination velocity, and which, in turn, impedes minority carrier device operation, particularly in high voltage applications. High temperature processing also limits the construction of abrupt junction structures within the monocrystalline region. Thus, although silicon dioxide passivation of a monocrystalline silicon semiconductor region enhances semiconductor technology, it also limits the ultimate device performance in many instances.
Additional passivation techniques have been considered in attempts to overcome the silicon dioxide limitations. It has been proposed, for instance, to deposit silicon nitride films by low pressure chemical vapor deposition (LPCVD) or plasma assisted techniques over silicon dioxide passivation films. The silicon nitride films are hard, tend to be immune to alkali and halide ionic penetration, and provide a moisture barrier, but tend to crack easily, reducing their effectiveness as a passivation, and are sensitive to charge reflection. Excessive stresses result from the nitride film, and morevoer, techniques proposed to alleviate cracking tend to exacerbate the stress differentials.
The addition of organic films, such as polyimide, has been proposed. While organic films impede ionic penetration and are basically void of stress, they are generally a poor termination for a monocrystalline lattice, and are susceptible to moisture penetration.
Undoped polycrystalline silicon has been proposed to passivate P-N junctions. The undoped polysilicon tends to create a high surface recombination velocity that opposes a standing inversion layer and tends to laterally distribute any electric field associated with a reverse biased junction. However, undoped polysilicon passivation tends to be excessively leaky.
In addition, oxygen doped polycrystalline silicon films have been employed to reduce the leakage and to lower the surface recombination velocity, but are very sensitive to oxygen, moisture, and temperature effects.
Furthermore, a triple film passivation has been proposed comprised of undoped polysilicon, oxygen doped polycrystalline silicon, and silicon nitride. The triple sandwich, as proposed, however, does not permit adequate control of the surface Fermi potential by virtue of the process technique, and is subject to stress effects between the nitride and the oxygen doped polycrystalline silicon.